Small electronic devices, such as multifunctional mobile phones (smartphones), multifunctional mobile terminals (tablet computers), car navigation systems, mobile game machines, or electronic dictionaries, office automation (OA) or factory automation (FA) equipment, and other devices have a display including a capacitance touch sensor in its display screen to be capable of receiving inputs through the screen. Such a touch sensor includes an electrode, which is required to be transparent. The electrode is made of an electrically-conductive transparent film.
Examples that have been used thus far as a material of an electrically-conductive transparent film include indium-tin-oxide (ITO), indium oxide, and tin oxide, which have high visible-light transmittance. Mainstream electrodes in touch sensors are formed by patterning electrically-conductive transparent films made of the above-described material.
A typical method for patterning an electrically-conductive transparent film includes forming an electrically-conductive transparent film, then forming a resist pattern on the film by photolithography, and removing a predetermined portion of the electrically conductive film by wet-etching to form an electrically conductive pattern. In the case of an ITO or indium oxide film, a mixture of two liquids, hydrochloric acid and ferric chloride, is typically used as an etching liquid.
An ITO film or a tin oxide film is typically formed by sputtering. This sputtering, however, is likely to change the properties of the electrically-conductive transparent film depending on the types of sputtering, the power of sputtering, the pressure of gas, the temperature of the base film, or the types of the atmosphere gas. The difference in film qualities of electrically-conductive transparent films caused by the changes of the sputtering conditions causes variations of the etching speed at which the electrically-conductive transparent film is wet-etched and is more likely to reduce product yield due to patterning defects. The above-described method for forming an electrically conductive pattern involves a sputtering step, a resist-forming step, and an etching step. Thus, it takes a long time to finish these steps and its cost is a large burden.
A trial conducted these days to solve the above-described problems is to form a transparent electrically conductive pattern from a material that replaces a material such as ITO, indium oxide, or tin oxide. For example, PTL 1 below discloses a method for forming an electrically conductive pattern by forming, on a base film, an electrically conductive layer containing an electrically conductive fiber such as a silver fiber, then forming a photosensitive resin layer over the electrically conductive layer, and exposing the photosensitive resin layer to light through a pattern mask disposed on the photosensitive resin layer to develop the photosensitive resin layer.
PTL 2 below discloses the following method for forming an electrically conductive pattern. A transfer photosensitive electrically conductive film, including a laminate of a photic resin layer and an electrically conductive layer containing an electrically conductive fiber such as a silver fiber, is formed over a support film. This film is laminated on a base film such that the photosensitive resin layer is tightly attached to the base film. The photosensitive electrically conductive film is exposed to light through a pattern mask disposed on the photosensitive electrically conductive film. After the support film is removed, the photosensitive electrically conductive film is developed.